1. Field of the Invention
The present invention relates generally to liquid scintillation counting and, more particularly, to methods of correcting measurements of a liquid scintillation sample for sample quench.
2. Description of the Prior Art
Liquid scintillation counters are widely used to measure the number and energy levels of radioactive emissions from a substance. In liquid scintillation counting a radioactive sample is combined with an organic scintillation solution, and the resulting mixture constitutes the liquid scintillation sample analyzed by the instrument. The scintillation solution emits burst of light called scintillations in response to each radioactive emission from the radioactive sample. The counter measures the intensity or pulse height of each scintillation to determine the corresponding energy of the radioactive emission causing the scintillation, and counts the number of scintillations in selected pulse height channels or windows. The liquid scintillation counter thereby develops a pulse height spectrum for the liquid scintillation sample comprising a profile of numbers or count rate of scintillations vs. intensity or pulse height of the scintillations (energy).
It is well known that the phenomenon of quenching in a liquid scintillation sample operates to reduce the intensity and number of detected scintillations and that such reduction in detected scintillations increases as quench increases. The result of quench, therefore, is to shift the pulse height spectrum of the quenched liquid scintillation sample along the pulse height axis to lower pulse height values, and this is commonly referred to as "pulse height shift".
In order to correct for the effect of sample quench, systems have been developed for determining the degree of quench in a sample and for adjusting the relative position of the pulse height spectrum and the window in which sample scintillations are measured by an amount corresponding to the degree of sample quench. Such automatic quench compensation methods, in effect, operate to re-establish the correct relative position of the pulse height spectrum and the measuring window. See, for example, U.S. Pat. No. 4,029,401 assigned to the assignee of the present invention, wherein the correct relative position of spectrum and measuring window is established by adjusting the gain of the multiplier photodetector which detect the light scintillations or by adjusting the actual window settings in which the sample is measured or counted. With such systems, a sample may be initially counted without quench correction and the developed pulse height spectrum stored in a suitable memory for later retrieval. Then quench correction can be performed by adjusting the window settings within which sample count information is outputted or retrieved from the stored spectrum for analysis.
Measurement of the degree of sample quench for use in the foregoing quench compensation methods can be performed by any of numerous known techniques. See Horrocks, D. L., "Applications of Liquid Scintillation Counting" (1974), Ch. X. Academic Press. A more recent and highly desirable quench determination method, termed the "H-number technique", is disclosed in U.S. Pat. No. 4,075,480, also assigned to the assignee of the present invention. In the H-number technique, a liquid scintillation sample is irradiated by a standard source (e.g. cesium.sup.137) to produce a Compton scattered pulse height spectrum. The relative shift of a unique point on the leading edge of the Compton spectrum between the irradiated quenched sample and a similarly irradiated standard sample provides a measure of the sample quench.
Implementation of the foregoing quench correction method has assumed that the sample quench produces a shift in the Compton spectrum of the irradiated quenched sample which corresponds to or is proportional to the shift in the spectrum of the same quenched sample when not irradiated. Unfortunately, however, it has been found that such assumed correspondence does not exist across the full range of scintillation intensity or pulse height values.
With respect to the foregoing, two different phenomenon appear to play a part. First, it is known that the energy response of an unquenched liquid scintillation solution is nonlinear at the low energy end of the absolute energy vs. pulse height relationship. See Horrocks, D. L. "Pulse Height-Energy Relationship of a Liquid Scintillation for Electrons of Energy Less Than 100 keV", Nuclear Instruments and Methods, 30 (1964), pp. 157-160. In this regard, at the low energy end of scintillation intensities, the light output per unit energy input increases for reasons not yet fully understood. The effect of this phenomenon is to produce a pulse height response for a given low energy which exceeds the response predicted from an extrapolation of the high energy responses of the energy vs. pulse height relationship to lower energies.
A second and independent phenomenon takes place at lower scintillation intensities as a result of the minimum energy which must be present to produce a measurable response in a multiplier photodetector. At higher quench levels, some radioactive emissions may not produce enough light to be detected. The result is that only the scintillations of the higher energy portion of the spectrum are detected. Consequently, the shape of the pulse height spectrum will remain relatively unchanged with further increases in quench, but the number of events detected will decrease.
The effect of the two foregoing phenomena, therefore, is to reduce the extent or the relative amount of the sample pulse height shift at the lower values of scintillation intensities typically encountered at the high sample quench levels.
The problem in sample measurement posed by the foregoing arises because quench determination of an irradiated sample and actual measurement of the sample when not irradiated are derived at different scintillation intensities or energy levels. For example, in U.S. Pat. No. 4,075,480, quench determination is based on the relative shift of the leading edge of the Compton spectrum of the irradiated sample at relatively high scintillation intensities. However, the unirradiated sample, particularly if highly quenched, produces a spectrum at, and is measured at, considerably lower scintillation intensities. As indicated previously, the spectrum shift produced by quench at low energy levels is less than that produced at the higher levels. Since the quench correction is based on the extent of spectrum shift at the higher energy levels, the correction thus derived will, in effect, "over-correct" for the actual effect of quench on the unirradiated sample at lower energy levels.